PTOs are used on agricultural vehicles such as tractors to provide power for equipment or implements such as combines, mowers and spreaders. As the use of PTOs developed, most tractor manufactures standardized on 1000 RPM and 540 RPM PTOs. This standardization involved the use of a common size shaft and spline arrangement for each RPM rating. When the shaft sizes were standardized years ago, tractors had relatively low horsepower (e.g. 30 to 50 horsepower). Accordingly, the torque output of a PTO shaft was limited by the horsepower of the tractor.
Modern tractors commonly have horsepower ratings in excess of 100 horsepower. However, the shaft sizes for PTOs have not changed due to the need to maintain compatibility with older equipment and maintain the standardization for PTOs. Thus, the torque output of PTOs for many modern tractors is no longer limited by the tractor horsepower. Rather, the torque output is limited by the strength of the PTO shaft and the failure thereof. For the very high horsepower tractors (e.g. over 130 horsepower) manufacturers have eliminated the 540 RPM PTO shaft. Due to the gear reduction required to achieve a PTO speed of 540 RPM at engine idle, the very high horsepower tractors can apply a torque to the 540 RPM shaft in excess of that required for the shaft to fail.
In addition to causing PTO shaft failures, the torque produced by the higher horsepower tractors can accelerate equipment attached to the respective PTO at a rate which can damage the equipment. In an attempt to limit acceleration of the PTO shaft, PTO clutch controls have been used to monitor the speed of the input and output shafts of the PTO clutch. Based upon the monitored speeds, the controls turn the clutch control valve ON and OFF in a cyclic fashion to limit the rate at which the PTO shaft is accelerated. However, since PTO clutch control valves are typically either fully ON or OFF, the acceleration of the PTO shaft occurs in a step-wise manner which may in itself introduce undesirable torque pulses into the PTO shaft and associated implement.
In view of the problems involved in the control of PTO shafts in high horsepower tractors, it would be useful to provide a PTO clutch control system for protecting PTO shafts from catastrophic failure, and providing PTO shaft accelerations at rates which protect the shafts and attached implements during clutch engagement.